The invention relates to a biaxially oriented polyolefin film which can be sealed on both sides and is composed of three layers, the base layer being composed essentially of propylene polymers and the two sealing layers being essentially composed of sealable olefin polymers. The film is distinguished by universal applicability to high-speed packaging machines and is also readily printable. The invention also relates to a process for the preparation of the film and to its use.
European Patent Application No. 0,008,904 discloses a biaxially oriented, three-layer polyolefin film which can be sealed on both sides and in which the base layer is formed by propylene polymers and the two sealing layers (outer layers or top layers) are formed by sealable olefin polymers. Although this polyolefin film has good heat-sealability, it has, in particular, only a low scratch resistance, cannot be printed and also leaves much to be desired with respect to transparency and slip properties in regard to high-speed packaging machines.
U.S. Pat. No. 4,502,263 discloses a biaxially oriented polyolefin film composed of three layers which is transparent and particularly readily sealable. Its base layer is composed essentially of a propylene polymer, and the two sealing layers are composed of an appropriate olefin polymer containing, as additives, 5 to 15% by weight of a low-molecular resin which is compatible with the olefin polymer, 5 to 15% by weight of a propylene homopolymer and 0.3 to 1.5% by weight of polydiorganosiloxane, the percentages by weight relating in each case to the sealing layer. This polyolefin multi-layer film possesses characteristics which are important for packaging films, namely a wide range of sealing, a low sealing temperature, high gloss and good transparency, a relatively high scratch resistance and low friction and hence good running characteristics on high-speed packaging machines of various types. However, it lacks a characteristic which is also important, namely good printability.
A biaxially oriented polyolefin film which can be sealed on both sides and has three layers is also disclosed in U.S. Pat. No. 4,419,411. In the case of this film too, an additive combination is incorporated in both of the sealing layers. They each contain 0.15 to 0.4% by weight of polysiloxane and 0.05 to 0.5% by weight of silicon dioxide, the percentages by weight relating in each case to the sealable layer. The base layer is composed of polypropylene (as the main component) and contains a small amount of a monocarboxamide. Part of the amide migrates from the base layer into the two sealing layers and onto the surface thereof (external face), so that polysiloxane, silicon dioxide and the monocarboxamide are present in incorporated form in each sealing layer, and the monocarboxamide is also present on the outer faces of the two sealing layers. The polyolefin multi-layer film described is stated to have a particularly low coefficient of friction. However, this film too suffers in particular from the disadvantage that it is not printable.